Harmful compounds such as nitrogen compounds known as NOx contained in an exhaust gas discharged from a coal-fired boiler for power generation or an incinerator are conventionally decomposed with a metal oxide having a denitration catalytic activity such as TiO2 or the like. As a catalyst carrying such a catalytic component, a catalyst having a catalytic component such as TiO2 carried on a surface of a carrier structure with a plate-like structure, a honeycomb-like structure, etc. has been used.
However, in the above-mentioned exhaust gas, other than harmful compounds such as nitrogen compounds, a large amount of dust is contained. When such a large amount of dust comes into contact with a catalytic surface, the catalytic component carried on the catalytic surface is abraded, and therefore, the catalytic component on the surface is decreased over time. Due to this, it was difficult to apply such a catalyst for a long time. This problem is evident on a catalytic end face where the collision frequency between dust in the exhaust gas and the catalyst is high.
Studies in which the amount of the catalytic component carried on the catalytic surface is increased and the like were performed to cope with the decrease in the catalytic component, however, a significant effect was not observed. Therefore, the development of an excellent catalyst in abrasion resistance in which the catalytic component carried on the catalytic surface is not decreased even if an exhaust gas containing a large amount of dust is brought into contact with the surface has been demanded. As one of such catalysts, a catalyst whose catalytic end face having a high collision frequency with dust has been strengthened by coating is described in Patent document 1.
However, although the above method of strengthening the catalytic end face is effective in the case where the density of the catalytic component on the catalytic surface is equal to or higher than a given value, when this density is lower than the given value, a decrease in the catalytic component on the end face due to abrasion is observed. This reason is that voids between catalytic component particles are increased when the density of the catalytic component is decreased, and therefore, even if an aqueous metal salt solution as an end face hardening agent is introduced, the increased voids cannot be filled. Further, even if the catalyst is dipped in an aqueous metal salt solution as an end face hardening treatment and thereafter the catalyst is dipped in the aqueous metal salt solution again without performing a drying step, the voids in the catalyst are filled with the aqueous solution in the first dipping, and therefore, the voids in the catalyst are not further filled with the aqueous metal salt solution in the second dipping. Accordingly, the abrasion strength of an end face cannot be improved by this method.
Patent document 1: JP-A-2001-170491